1. Technical Field
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method for providing a good diagnostic image from which an artifact caused by an uneven static magnetic field and a nonlinear gradient magnetic field is removed.
2. Background Art
An MRI apparatus has a static field generation magnet formed about an axial center in an advancing/retreating direction of a table-top, a shim coil formed inward of the static field generation magnet about the axial center in the advancing/retreating direction of the table-top, gradient coils formed inward of the shim coil about the axial center in the advancing/retreating direction of the table-top, and a liner set on inner side of the gradient coils about the axial center in the advancing/retreating direction of the table-top, and forming a bore through which the table-top advances and retreats. During imaging, the MRI apparatus generates a static magnetic field in the bore formed by the liner, and the gradient coils form gradient magnetic fields in X-, Y-, and Z-directions in an imaging area of a patient set in the bore. Further, the MRI apparatus causes a nuclear spin in the patient to magnetically resonate by transmitting an RF signal from an RF (high-frequency) coil provided with the MRI apparatus, and reconstructs an image in a reconstruction FOV (field of view) in the patient making use of an NMR (nuclear magnetic resonance) signal generated by an excitation.
However, an artifact is generated to a reconstructed image due to an uneven static magnetic field and nonlinear gradient magnetic fields. To cope with the above problem, there is disclosed a technology for correcting an artifact on a reconstructed image caused by an uneven static magnetic field (refer to, for example, Japanese Patent Application Publication 2006-61235).
In contrast, to correct an artifact on a reconstructed image caused by the intensities of gradient magnetic fields with the nonlinearity, there is known a technology for previously providing a position coordinate correction table for causing the distribution of the intensities of actual gradient magnetic fields with the nonlinearity to agree with the distribution of the linear intensities of virtual gradient magnetic fields and correcting a reconstructed image using the correction table. Conventionally, since a display FOV (in a displayed image) that is a display area of a display image is small regardless whether a bore diameter is relatively narrow (for example, 600 mm) or relatively wide (for example, 700 mm), an artifact is corrected well even if a reconstruction FOV (in the patient) approximately agrees with the display FOV (in the image).
However, when the bore diameter is relatively large and the display FOV is relatively large, the reconstruction FOV must be widened more than the display FOV to appropriately correct an artifact generated on a side away from the center of a magnetic field (center of the reconstruction FOV) in the display FOV. This is because since the intensities of the actual gradient magnetic field become more nonlinear as they are away from the center of the magnetic field, the information of position coordinates on a side away from the center of the magnetic field in the display FOV must be supplemented by the information of position coordinates externally of the display FOV.
However, when the reconstruction FOV is widened, a part, in which the distribution curve of the intensities of the gradient magnetic fields is shown by a two-value function, appears in the reconstruction FOV. In this case, according to the conventional technology, since a plurality of position coordinates in the intensities of actual gradient magnetic fields correspond to one position coordinate in the display FOV in the intensities of virtual gradient magnetic fields, an appropriate correction table cannot be generated. In particular, since a bore diameter and a display FOV tend to enlarge now, it is desired to appropriately correct a reconstructed image because it is expected that an artifact is liable to be generated on a reconstructed image. When an artifact exists on a display image, it may be an obstacle for diagnosis.